This invention relates to molded articles of fiber reinforced synthetic plastics material and to a process of molding such articles.
Plastics sheet materials made from thermoplastic resins are widely used in the manufacture of molded articles. Such materials are, however, not of great strength or rigidity, and where such properties are required, fiber reinforcement is introduced.
Thus, for example, in the manufacture of one such materials, layers of glass fiber mat are interposed between layers of thermoplastics material, the composite structure being needled to produce a degree of integration of the layers and then heated under pressure to produce consolidated rigid sheets for use in molding.
For the satisfactory molding of such sheets, they must be homogeneously preheated. This requires both time and accurate temperature control if overheating and degradation of the sheet surfaces is not to occur whilst the core portions of the sheets are brought up to the required molding temperature. Such materials do not lend themselves easily to deep draw molding.
Also, for a molding of given dimensions, an optimum size of consolidated sheet is required if excessive waste in the form of flash is to be avoided. As a result a molder who manufactures a wide range of moldings must carry a corresponding range of sheet sizes, be prepared to cut large sheets to an appropriate size or accept a high degree of wastage.
Furthermore, when used for deep draw molding it is found that such materials are not capable of being used to form moldings of uniform structural strength. This is because the glass fiber mat is constituted of very long glass fiber strands (i.e. fiber bundles) of perhaps 200 centimeters or more which extend in a random serpentine manner throughout the whole sheet. This substantially restricts their movement during molding in that they cannot flow with the thermoplastics material constituting the remainder of the structure. As a result, relatively thin parts of the molding such as stiffening ribs are starved of fiber reinforcement. Additionally, because of the mode of manufacture of such reinforced sheets, they have to be fully consolidated by the application of heat and pressure in order to be transportable. As a result, they can only be supplied to the molder as flat, impermeable and rigid sheets which are difficult to handle in a continuous molding process.
It is among the objects of the present invention to provide a composite fiber and plastics material and process for the molding of fiber reinforced plastics articles which overcomes or alleviates the disadvantages of known materials as described above.
U.S. continuation application Ser. No. 689,000, now abandoned, describes and claims an air permeable sheet-like structure consisting essentially of 20% to 60% by weight of single discrete reinforcing fibers having a modulus of elasticity higher than 10000 Mega Pascals, and being between about 7 and about 50 millimeters long, and 40% to 80% by weight of unconsolidated particulate plastics material, selected from the group consisting of a thermoplastic and a thermosetting plastic material, the particulate plastics material having a particle size less than about 1.5 millimeters, and in which the fibrous and plastics components are bonded into an air permeable structure with the particulate plastics material retaining its particulate form in the air permeable structure, and the subject matter disclosed in this application Ser. No. 689,000, now abandoned, is incorporated by reference herein.
According to the present invention a process for making a shaped article of fiber reinforced synthetic plastics material comprises the steps of
forming a foamed aqueous dispersion comprising from 20% to 60% by weight of single discrete fibers having a modulus of elasticity higher than 10000 mega pascals, and between 7 and 50 millimeters long, and from 40% to 80% by weight of unconsolidated particulate plastics material selected from the group consisting of a thermoplastic material and a thermosetting material, the particulate plastics material having a particle size of less than about 1.5 millimeters; PA1 laying down and draining said dispersion on a foraminous support so as to form a web; PA1 transferring said web to a heating oven; PA1 heating said web first so as to remove residual moisture therefrom and then so as to bond the fibrous and plastics components together into a self-sustaining permeable sheet while substantially maintaining the particulate form of the plastics material; PA1 transferring said self-sustaining sheet to a through air heating oven; PA1 passing heated air through said sheet so as to cause substantially uniform and homogeneous heating of the components thereof to a temperature at which the viscosity of the thermoplastic constituting the particulate component is sufficiently low to permit the sheet to be molded into a shaped article; PA1 transferring said heated permeable sheet to a compression mold; and, PA1 subjecting said sheet to compression molding at a predetermined pressure so as to form a shaped fiber reinforced plastic article. PA1 forming a foamed aqueous dispersion comprising from 20% to 60% by weight of single discrete fibers having a modulus of elasticity higher than 10000 megapascals, and between 7 and 50 millimeters long, and from 40% to 80% by weight of unconsolidated particulate plastics material selected from the group consisting of a thermoplastic material and a thermosetting material, the particulate plastics material having a particle size of less than about 1.5 millimeters; PA1 laying down and draining said dispersion on a foraminous support so as to form a web; transferring said web to a through air heating oven; PA1 passing heated air through said web, first so as to remove residual moisture and then so as to cause substantially uniform and homogeneous heating of the components thereof to a temperature at which the viscosity of the thermoplastic constituting the particulate component is sufficiently low to permit the web to be molded into a shaped article; PA1 transferring said heated web to a compression mold; and, PA1 subjecting said web to compression molding at a predetermined pressure so as to form a shaped fiber reinforced plastic article.
The heating oven can be in the form of a through air heating oven.
Where glass fibers are used, and are received in the form of chopped strand bundles, the bundles are broken down into single fibers before the structure is formed.
A high modulus of elasticity is to be taken as meaning a modulus of elasticity substantially higher than that of a consolidated sheet which could be formed from the structure. Fibers falling into this category include glass, carbon and ceramic fibers and fibers such as the aramid fibers sold under the trade names Kevlar and Nomex and will generally include any fiber having a modulus higher than 10,000 Mega Pascals.
Bonding may be effected by utilizing the thermal characteristics of the plastics material within the structure. Thus the structure may be heated sufficiently to cause a thermoplastic component to fuse at its surfaces to adjacent particles and fibers. Or a post formable thermosetting component may be so heated to produce a similar effect. Care must be taken however to ensure that the conditions of heating are such as to prevent degradation of the plastics material after bonding.
Alternatively, a binder may be added during manufacture of the structure to effect bonding. Any binder may be used which will effect a bond at a lower temperature than that which would result in consolidation of the plastics material within the structure. Suitable binders include polyvinyl alcohol, polyvinyl acetate, carboxymethyl cellulose and starch.
Individual fibers should not be shorter than about 7 millimeters, since shorter fibers do not provide adequate reinforcement in the ultimate molded article. Nor should they be longer than 50 millimeters since such fibers are difficult to handle in the preferred manufacturing process for the fibrous structure.
Preferably glass fibers are 13 microns in diameter or less. Fiber of diameters greater than 13 microns will not so efficiently reinforce the plastics matrix after molding.
Preferably, the plastics material is in a particulate form and may be a thermoplastic, a thermosetting plastic or a mixture of the two. Suitable thermoplastics include polyethylene, polypropylene, polystyrene, acrylonitrylstyrene butadiene, polyethylene terephthalate, and polyvinyl chloride, both plasticized and unplasticized. It is anticipated that any thermoplastics powder may be used which is not chemically attacked by water and which can be sufficiently softened by heat to permit fusing and/or molding without being chemically decomposed.
Plastics powders need not be excessively fine, but particles coarser than about 1.5 millimeters, as exemplified by coarse sand or fine rice grains, are unsatisfactory in that they do not flow sufficiently during the molding process to produce a homogeneous structure.
The use of larger particles results in a significant reduction in the flexural modulus of the material when consolidated. Preferably the plastics particles are not more than 1 millimeter in size.
Because the structure is permeable, it is capable of being preheated by hot air permeation. This technique permits rapid homogeneous heating of the whole structure in a manner which is difficult to achieve with consolidated sheets.
The degree of bonding can be controlled to cohere the components whilst still retaining sufficient flexibility to permit the structure to be reeled. In the reeled condition, it can be transported readily for use to the mold in a continuous preheating and molding process. Alternatively, and to minimize material wastage, shaped elements may be cut, pressed or stamped from the structure and supplied to the mold in a form permitting articles to be molded with minimum flash to be disposed of. The residual material may be recycled through the forming process, and neither the molder nor the manufacturer of the fibrous structure will be faced with the need to dispose of waste material.
Alternatively, the degree of bonding may be such as to produce a rigid, but still air permeable sheet where this will meet the molder's requirements. This is effected by adjusting the amount of the degree of fusing of the thermoplastic, or the amount of binder added to achieve the desired effect, the adjustment depending on the kinds of thermoplastics or binders used.
After preheating, the structure may simply be molded into an impermeable article. Alternatively, it may be subjected to limited compression in the mold so as to remain permeable. Or it may be fully compressed in the mold so as to cause the molten thermoplastics material to wet the fibers. The mold is then slightly opened so as to allow the material to expand as a result of the resilience of the fibers and become permeable as described and claimed in U.S. Pat. No. 4,670,331 the subject matter discussed in that Patent being incorporated by reference herein. In certain cases, and especially when a smooth or glazed surface finish is required, the structure may be impregnated with a liquid thermosetting resin before or after molding as described and claimed in U.S. Pat. No. 4,690,860 the subject matter disclosed in that Patent being incorporated by reference herein.
The porosity of the structure permits the optional introduction of liquid thermosetting resin by surface coating or impregnation. Such resins must, of course, be of the slow curing or post formable kind so as to permit delivery to the molder and molding before curing occurs.
The structure may be rapidly heated by the heated air to the molding temperature of the thermoplastic component. The sheet will then be quickly transferred to the molding press and pressed into the desired shape before the curing of the thermosetting resin is complete.
The impregnation may be complete, in which a dense article will result or it may be limited to the surface layers of the article. This may confer sufficient increase in stiffness over the original expanded thermoplastic, together with a sealed surface which prevents a further ingress of other fluids such as water or oil into the expanded central zone. An excess of liquid thermosetting materials on the surface may also be used to produce a very smooth glossy appearance which is desirable when the molding is to be used as a substitute for sheet metal and which is very difficult to achieve with conventional fiber reinforced materials.
Thermosetting resins which may be used to impregnate the expanded thermoplastics sheet include phenolic and polyester resins, for example phenol-formaldehyde resin, urea and melamine formaldehyde resins, epoxy resins, unsaturated polyesters and polyurethanes. Post formable thermosetting materials may also be used.
In those cases where the molder is only equipped to handle consolidated sheets, the fibrous structure may be consolidated by cutting into appropriate lengths and then heating and cooling under pressure. It will be appreciated that such consolidation can only be carried out when the plastics content of the sheet is wholly of thermoplastics material.
In all cases however, after the web has been formed it is treated, usually by heating, to effect bonding without substantially consolidating the plastics particles held in the web. Slight metering may be effected to ensure that the structure produced has a constant thickness. However, pressure and temperature conditions must be less than those which would compact the web and consolidate any thermoplastic component or cure any thermosetting component which it may contain.
The invention also includes a process for making a shaped article of fiber reinforced synthetic plastics material comprising the steps of
A convenient process according to the invention includes heating the sheet in an oven in which the sheet is located between open supports through which hot air is passed.